Earth Barely Big Enough for Life, Study Says

Astronomers searching for habitable worlds might do best to look for rocky planets several times larger than Earth.

That's because, according to a new study, our planet is at the lower end of the size range needed for plate tectonics—which scientists believe are vital for stabilizing temperatures enough for life.

Tectonics—the continent-shifting forces that build mountains and fuel volcanoes—recycle Earth's crust by drawing it underground, where it melts and later re-emerges as magma, pointed out Diana Valencia of Harvard University.

That helps keep carbon dioxide levels in the atmosphere more or less stable, as excess gas is removed from the atmosphere by reacting with fresh rocks in a process called "weathering."

The carbon dioxide is later returned to the atmosphere via volcanic gases.

"Plate tectonics is important for the carbon cycle to operate," Valencia said. "Carbon cycling is the Earth's climate thermostat. Over time, it's kept the Earth's temperature around [that of] liquid water, allowing life to emerge."

Valencia, who presented her study yesterday at a meeting of the American Astronomical Society in Austin, Texas, and her team looked at factors that determine whether rocky planets experience tectonics.

Rocky worlds significantly larger than Earth—commonly known as "super-Earths"—are more likely to have plate tectonics, the researchers concluded.

"It gives us hope" for finding habitable worlds, Valencia said. "Finding an Earth analog is going to be hard. But finding super-Earths is easier."

The pair concluded that the old stars were forming new planets long after their first generations of planets had matured.

The most likely reason for the phenomenon is that some other object, perhaps a small star or large planet, had recently been drawn into each stars' atmosphere, he added.

"It may still be there, burrowing through the outer reaches of the atmosphere," Zuckerman said.

"The gravitational interaction is enough to toss material out into this gaseous ring where this second generation of planets might form. People have been writing theoretical papers about the formation of such gaseous rings for a long time. This may be the first observation."

It's unlikely that such late-forming planets would support life. But "it would be interesting to see what kind of planets form in such an environment, and what their characteristics would be," Zuckerman said.

Harvard's Valencia said it's an interesting find.

"If it's the case that you have [that] second wave," she said, "you would have even more interesting and different planets."